A vinyl chloride-based polymer is a homopolymer of vinyl chloride or a copolymer including 50% or more of vinyl chloride, wherein it has been widely used in areas, such as piping materials and building materials, due to excellent mechanical strength, weather resistance, and chemical resistance. However, since the vinyl chloride-based polymer by itself has low moldability, various additives such as a plasticizer, are appropriately added to provide processability.
In general, the moldability of the vinyl chloride-based polymer largely depends on particle characteristics of the vinyl chloride-based polymer. Also, in order to improve productivity during the preparation of a molded article using the vinyl chloride-based polymer, there is a need to improve gelling properties, plasticizer absorbency, or powder flowability by controlling bulk density or internal porosity, a particle diameter, or particle distribution of vinyl chloride-based polymer particles.
As a typical method of improving the moldability of the vinyl chloride-based polymer, a method of improving processability of a vinyl chloride polymer itself by using a dispersion stabilizer such as polyvinyl alcohol, during polymerization of the vinyl chloride-based polymer has been proposed. However, according to the above method, bulk specific gravity of the vinyl chloride-based polymer may be increased, but melting characteristics of the polymer may be deteriorated.
Accordingly, as a method of improving the melting characteristics as well as the processability of the vinyl chloride-based polymer, a method of controlling distribution of polymerization degree by changing temperature during the polymerization of the vinyl chloride-based polymer has been proposed. However, according to the above-described methods, a vinyl chloride-based polymer having high bulk density as well as improved processability may be prepared, but productivity may be reduced due to an increase in polymerization reaction time, and deterioration of physical properties of the polymer itself, for example, an increase in colorability or a decrease in mechanical properties, may occur due to a side reaction between an additive and a reactant such as a vinyl chloride-based monomer, according to the changes in the polymerization temperature.
As another method of improving the moldability of the vinyl chloride-based polymer, a method of using a plasticizer with the vinyl chloride-based polymer during the preparation of a molded article has mainly been used. However, when the plasticizer is used as described above, since the plasticizer may move to a surface of the product, stickiness of the surface may occur and plasticity may be gradually reduced over time. In particular, with respect to a phthalate-based plasticizer mainly used, a lot of smoke may be generated during combustion, flexibility may occur at high temperature, and high energy may be required during processing
Also, the appearance characteristics of molded articles are considered very meaningful together with the improvement of molding technique or mixing technique, and there is a need to develop a technique which may decrease the generation of fish-eyes and improve transparency.
For example, U.S. Pat. No. 5,204,421 discloses a preparation method in which a dispersant having a degree of hydration of 20% to 55% is added in installments in the initial stage and the middle stage of polymerization, and indicates that a vinyl chloride polymer having excellent plasticizer absorption and a small number of fish-eyes can be prepared through the preparation method. U.S. Pat. No. 7,001,960 discloses a preparation method in which a dispersant having a degree of hydration of 20% to 57% is continuously added from the initial stage of polymerization to the middle stage of polymerization, and indicates that a vinyl chloride polymer having a small number of fish-eyes can be prepared through the preparation method. Korean patent publication No. 2013-0001428 discloses a method for suppressing the formation of scale in a polymerization reactor and thus preventing the generation of fish-eyes by adding a metal deactivator during a polymerization reaction. However, the above conventional methods have a negligible effect on the reduction of fish-eyes and cannot significantly improve the transparency of produced molded articles.
Therefore, in order to easily apply a vinyl chloride-based polymer to various fields, it is necessary to develop a technique capable of improving the processability of the vinyl chloride-based polymer and the appearance characteristics of produced molded articles.